Conventionally, as a gate electrode of a GaAs MESFET having a self-aligned structure due to ion implantation, a refractory metal such as W (tungsten) or a refractory metal silicide such as WSi.sub.x (tungsten silicide) is widely used.
Among those, WSi.sub.x is superior in the characteristics of the Schottky junction formed with GaAs as well as stability against thermal processing after the formation of the junction, and is well applied as a gate electrode of a GaAs MESFET.
However, because WSi.sub.x has a high resistivity, for example, about 150 .mu..OMEGA..multidot.cm when x=0.3, when it is adopted as a gate electrode of a GaAs MESFET, the gate resistance would be unfavorably increased.
FIG. 6 is a diagram illustrating a cross-section of a GaAs MESFET disclosed in Japanese Published Patent Application No. Sho. 60-132375. MESFET has superior Schottky characteristics with WSi.sub.x and the stability against thermal processing after forming the junction, and it is quite effective as means for reducing the gate resistance.
In FIG. 6, reference numeral 12 designates a semi-insulating GaAs substrate, numeral 13 designates an active layer comprising n type GaAs. Numeral 14 designates a first metal layer comprising Ta.sub.x W.sub.y Si.sub.1-x-y, numeral 15 designates a second metal layer comprising Ta.sub.v W.sub.z Si.sub.1-y-z. Numeral 16 designates a gate electrode, numeral 17 designates a source electrode, and numeral 18 designates a drain electrode. The gate electrode 16 comprises a double-layer structure comprising the first metal layer 14 and the second metal layer 15.
In the above-described two-layer structure, the gate resistance of the gate electrode 16 becomes the smallest when the first metal layer 14 comprises WSi.sub.x and the second metal layer 15 comprises W. Here, the value of x represents the minimum composition ratio that can produce superior Schottky characteristics in the Schottky junction which is obtained with the first metal layer 14 comprising WSi.sub.x contacting the active layer 13 comprising n type GaAs.
However, when the first metal layer 14 comprising WSi.sub.x and the second metal layer 15 comprising W are formed on the semi-insulating GaAs substrate 12, the internal stress arising in the first metal layer 14 and the second metal layer 15 becomes large, and as a result, it is likely to cause peeling at the interface between the first metal layer 14 and the active layer 13 on the semi-insulating GaAs substrate 12, thereby resulting in difficulty in forming the gate electrode stably.